Conventional television receiving systems conform to standards, such as NTSC and PAL, which have evolved since the initiation of television broadcasting. Since these standards evolved over time, they include, for example, modulation schemes that allow for the transmission of color images without adversely affecting the reception and reproduction of these images on televisions that are only capable of displaying images in black and white. These standards also include the encoding of closed caption information within each frame, also formatted so as not to adversely affect the reception and reproduction of images on televisions that are not capable of displaying closed caption text.
Standards, such as SMPTE 125 and CCIR 601, have been developed for the digital production, storage, and display of images that are substantially compatible with the display of images using NTSC and PAL rasterizing techniques, albeit at higher resolution. These standards address the number of lines per screen image (vertical resolution), the number of digital samples per line (horizontal resolution), and the number of bits per digital sample. The encodings provided by these standards are baseband encodings, and do not include, for example, the modulations that are applied for conventional NTSC or PAL raster encodings with embedded closed caption information. To communicate a digital encoding of image data to a conventional television receiver, the image data must be modulated to conform to the conventional television broadcast conventions, as defined for example by the FCC. That is, the FCC has allocated a maximum of 6 MHz for a television channel transmission and has requested the color information, or chrominance, to be quadrature-phase modulated about a chrominance subcarrier frequency at 3.58 MHz. The FCC has also requested the audio information to be frequency modulated about a sound center frequency at 4.5 MHz and the closed caption information to be contained on lines 21 and 284 of the channel transmission, preceded by a clocking signal of approximately 500 kHz. These frequencies are specific for the North American NTSC standard and are different for other standards and countries.
FIG. 1 illustrates a prior art technique for encoding image data 101, closed caption data 102, and other signals 143 for communication to a conventional raster television 160. In this example, an image processing system (not shown) creates digitally encoded image data 101. The image processing system may be, for example: a computer graphics system that is used to create images; a computer system that receives images from other sources, such as the world-wide web; a digital video disc player; and the like. The aforementioned digital image standard, CCIR 601, specifies an interface clock rate of 27 MHz for the communication of image data, corresponding to the sample rate of the image data, which consists of a luminance component at 13.5 MHz, and two chrominance components at 6.25 MHz each. The video encoder 130 encodes these samples for transmission to a raster television using conventional television broadcast encoding techniques, common in the art. The video encoder 130 produces raster encoded sample data 131 that is converted into analog form by a digital to analog converter 140 to form an analog video raster signal 141.
The closed caption encoder 132 separately processes the closed caption data to form a closed caption raster signal 142 that conforms to the particular rasterization standard used. For example, in accordance with FCC standards, lines 21 and 284 of the conventional NTSC television broadcast are allocated to closed caption. The closed caption information consists of two bytes per television frame; the two bytes of closed caption information must be preceded by a 500 kHz sinusoidal clock signal. The c losed caption encoder 132 provides the 500 Hz clock signal followed by two bytes of the closed caption data 102 as the closed caption raster signal 142.
The sync control 150 mixes the analog video raster signal 141, the closed caption raster signal 142, and any other raster signals, such as modulated audio information, teletext encoded messages, and the like. The sync control 150 identifies, for example, each 21.sup.st and 284.sup.th line of the composite raster image, and inserts the closed caption raster signal 142 at each such line. In addition, the sync control 150 adds the appropriate synchronization signals, such as the horizontal and vertical sync pulses, to form the composite raster television signal 151 that is communicated to a conventional raster television 160.
Conventionally, the processing of each form of data 101, 102 to form components 141, 142, 143 of the raster television signal 151 is performed independently, because each form of data 101, 102 has specific and independently formulated rules and standards. For example, the processing of image data is performed on discrete data samples of the image data 101, and produces modulated samples of the raster encoded data 231. The processing of closed caption data 102, on the other hand, is primarily the appending of an analog sinusoidal signal (not shown) to a subset (two bytes) of the closed caption digital data 102. This independent processing, however, results in redundancies in the development and production of the components in each of the independent processes, thereby decreasing the efficiency of the system and increasing the costs of devices and systems used to process image data 101 and closed caption data 102.
Consequently, there exists a need for an encoding system that combines the processing of closed caption data with image data to allow for a reduction in the size or cost and improved efficiency of such a system. In particular, a need exists for the concurrent processing of closed caption data with image data to allow for the time-shared use of a discrete time oscillator (DTO) for providing the closed caption clocking signal as well as the subcarrier signal that is used to modulate the chrominance components of the image data.